This ensures the metrics proxy endpoint is ACL protected behind a
wildcard `service:read` and `node:read` set of rules. For Consul
Enterprise these will need to span all namespaces:
```
service_prefix "" { policy = "read" }
node_prefix "" { policy = "read" }
namespace_prefix "" {
service_prefix "" { policy = "read" }
node_prefix "" { policy = "read" }
}
```
This PR contains just the backend changes. The frontend changes to
actually pass the consul token header to the proxy through the JS plugin
will come in another PR.
R.B. Boyer